Pneumatic controllers



N. BREWER ETAL 3,098,498

PNEUMATIC CONTROLLERS July 23, 1963 17 Sheets-Sheet 2 Filed June 8, 1953INVENTORS NATHAN/EL BREWER EDMUND D. HA/GLER BY EDWARD D. WOODRING E-WKW July 23, 1963 N. BREWER ETAL 3,098,498

PNEUMATIC CONTROLLERS Filed June 8, 1955 17 Sheets-Sheet 5 I28 I13 /24ll? I19 //5 //l I09 //0 INVENTORS NATHAN/EL BREWER EDMUND D. HAIGLEREDWARD D. WOODR/NG July 23, 1963 N. BREWER ETAL 3,098,498

PNEUMATIC CONTROLLERS Filed June 8, 1953 1'? Sheets-Sheet 4 I I immun IA/ 9 I w,

IN VE'N TORS F' ,1 NATHAN/EL BREWER '9 l8 EDMUND 0. HA/GLER I83 EDWARD0. WOODR/NG 21/ 2/3 BY #ICITTORNEY July 23, 1963 WE ETAL 3,098,498

PNEUMATIC CONTROLLERS Filed June 8, 1953 17 Sheets-Sheet 5 323 K; mm A1953303 308 g 322 290 29/ i 287 nMMMillannnnnnnnnnnnnnnm 262 Ffq. 29; i2e9 22.

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--- INl ENTORS NATHANIEL BREWER EDMUND D HA/GLER EDWARD 0, worm/v6 July23, 1963 N. BREWER ETAL 3,093,498

PNEUMATIC CONTROLLERS Filed June 8, 1953 17 Sheets-Sheet 6 WITH PENINVENTORS NATHAN/EL BREWER EDMUND D. HA/GLER EDWARD D. WOODR/NG' July23, 1963 N. BREWER ETAL PNEUMATIC CONTROLLERS 1'7 Sheets-Sheet 7 FiledJune 8, 1953 I/VVE/VTORS NATHAN/EL BREWER EDMUND D. HAIGLER EDWARD DWUODR/NG BY WfiW ATTQRLVEY /////I/IIIII July 23, 1963 N. BREWER ETAL3,098,498

PNEUMATIC CONTROLLERS Filed June 8, 1953 l7 Sheets-Sheet 8 INVENTORSNATHANIEL BREWER EDMUND D. HA/GLER EDWARD D. WOODR/NG BY July 23, 1963N. BREWER ETAL 3,098,498

PNEUMATIC CONTROLLERS Filed June 8, 1953 1'7 Sheets-Sheet 9 WITH F'RISING ou'rnu-r we Iu mcnuu:

1TH PEN msms 7 CU T PIISSIJII ":EI-Es INVENTORS V NATHAN/EL BREWEREDMUND D. HA/GLER EDWARD D. WOODR/NG ATTORNEY July 23, 1963 N. BREWERETAL PNEUMATIC CONTROLLERS Filed June 8, 1953 17 Sheets-Sheet l0INVENTORS NATHAN/EL BREWER EDMUND D. HA/GLER EDWARD D. WOODR/NG ATTORNEYJuly '23, 1963 N. BREWER ETAL PNEUMATIC CONTROLLERS l7 Sheets-Sheet 11Filed June 8, 1953 M ig gil EDWARD D. WOODR/NG EDMUND D. HA/GLERNATHAN/EL BRE WE R IN VE N TORS was I July 23, 1963 N. BREWER E .TAL3,098,498

PNEUMATIC CONTROLLERS Filed June 8, 1953 1'? Sheets-Sheet l2 /-57/NVENTOR5 NATHAN/EL BREWER EDMUND D. HA/GLER EDWARD D. WOODR/NG BY E-65. WK'W ATTORNEY July '23, 1963 N. BREWER ETAL PNEUMATIC CONTROLLERS l7Sheets-Sheet 13 Filed June 8, 1955 6 RN Y .HEWH E wmw m AO m RHW T B A s00 R wm f N UA mMw ADD WNEE Y B July '23, 1963 N. BREWER ETAL PNEUMATICCONTROLLERS 1'7 Sheets-Sheet 14 Filed June 8, 1953 mvE/vroRs NATHANIELBREWER EDMUND 0. HA/GLER EDWARD 0 WOODR/NG N. BREWER ETAL PNEUMATICCONTROLLERS July 23, 1963 Filed June 8, 1953 17 Sheets-Sheet 15 42.9 42845I 445 w 43/ 430 I36 432 7 49 440 48 a 427 429 44 45; 4/5 85 4/EmmnmlIllllllllllllllllllllllllllllllllI IHIHHIIIIIHIIIIII ll //V VE N T0R5 NAT HA /V/E L BREWER EDMUND 0. HA/GLER ED WA RD 0. WOOD/RINGATTORNEY July 23, 1963 N. BREWER ETAL 3,098,498

PNEUMATIC CONTROLLERS Filed June 8, 1953 1? Sheets-Sheet 16 I73 0 I65/74 M/ B i g 287 I6 250 INVENTORS 236 NATHAN/EL BREWER /a/ EDMUND 0.HA/GLER EDWARD 0. WOODR/NG ATTORNEY United States Patent 3,098,498PNEUMATIC CONTROLLERS Nathaniel Brewer, Newtown, Edmund D. Haigler,Hatboro, and Edward D. Woodring, Plurnsteadvrlle, Pa, assignors toFischer & Porter Company, Hathoro, Pa, a corporation of PennsylvaniaFiled June 8, 1953, Ser. No. 361,128 26 Claims. ((31. 137-86) Thepresent invention relates to controllers and particularly pneumaticcontrollers adapted to receive an input motion from a measuring element(such as that measuring rate-of-flow, temperature, pressure, humidity,llqllldlevel, viscosity, or specific gravity, etc); the 0utput of thecontrollers being a pneumatic or fluid pressure wh ch is impressed upona measuring element or upon a sett ng element in another indicating,recording or controlllng instrument or valve-positioning relay or upon afinal control element (such as an air-motor-operated valve, damper orthe like (whereby the rate-of-flow, temperature, pressure, humidity,liquid-level, specific gravity, or WISOO-SlllY is (then maintained atany desired and selected setting.

Thus, for instance, the input-motion of the controller of the presentinvention may be obtained from a rate-offlow meter, such as avariable-area type rate-of-fiowv meter (sometimes called rotameter)whose measurement or reading is first converted into a mechanical motionexternal of the metering chamber, by a magnetic couphng such as that(for example) shown in Brewer Patent 2,425,691, or Bowie Patent2,380,399, connected to one arm of a pivoted lever whose other arm isconnected to the input-lever of the present controller or firstconnected to the pen-arm of a recorder and from it connected to theinput-lever of the present controller, or whose measurement istranslated into mechanical motion through either a directelectro-magnetic coupling, such as for example, that of Brewer patentapplication Serial No. 106,171, filed July 22, 1949 (now Patent No.2,662,223, issued on December 8, 1953), or by means of an intermediateelectrical servo motor.

' Similarly, viscosity measurements, for example, from a viscosimeterlike that shown in Fischer Patent 2,426,393, may be impressed upon amagnetically coupled motiontransmitter or an electro-magneticallycoupled motiontransmitter, as above, or a single-float viscosimeter(namely, a variable-area type of metering chamber provided with a singleviscosity-sensitive float as shown, for example, in Brewer patentapplication Serial No. 300,164, filed July 22, 1952) may be inserted ina small by-pass from a pipe-line, in which lay-pass the rate-of-fl-ow ismaintained constant by means of a floWv-regulator-valve whose positionis controlled by a viscosity-immune rateof-flow meter like that (forexample) of Fischer Patent 2,350,343 coupled to and acting through avalve-regulator.

The pneumatic output of the controllers of the present invention may beimpressed upon a flow-regulating valve or damper, operated by apneumatic motor, which valve or damper is opened or closed, or whoseintermediate position is determined by the pneumatic output of thecontrollers of the present invention.

By pneumatic we intend to include not only airoperated but alsogas-operated controllers embodying the present invention.

One of the objects of the present invention is a sectionJal constructionin controllers such that with but comparatively few sections arelatively large repertoire of controllers may be formed and wherebysuch controllers may be made either right-handed or left-handed. Whilethe need for right-handed or left-handed controllers has specialreference to the side upon which the input lever is located, yet as theadjusting dial or dials on the conice troller are generally on the sideopposite to that on which the input lever or levers are located, werefer to a righthanded controller if the dial or dials are on therighthand side when facing the controller and we refer to it as aleft-handed controller if the dial or dials are on the left side.

Another object of the present invention is to achieve accuracy anddependability in the controllers through a rigid stack construction andalso to enable the more ready servicing and change of the controller inthe field to perform different functions, With a minimum number of partsto achieve all the various functions and combinations of function, andwith a minimum of general disassembly for specific service or conversionoperations.

One of the characteristics of the controllers of the present inventionis that they include sections which are symmetrical both as to theirpneumatic passageways as well [as to their fastening passageways aboutcertain planes of symmetry and are so arranged that the same housing andpneumatic-passageway-beaiing member may perform different functions inthe same controller by mere reversal of its position. Thus, forinstance, the same section performs the function of a top or closuremember as well as the function of a partition member orchamber-performing member in the lower part of the controller.Similarly, the same pneumatic section may be used above and below theinput-arrd-nozzle section for performing two different functions.

In the accompanying drawings, hereinafter referred to, like referencecharacters indicate like parts.

FIGURE 1 represents a perspective view of a B-function orproportional-resetderivative controller embodying the present invention,namely, a controller whose pneumatic output is substantiallyproportional to (a) the deviation of the measurement from thepredetermined setting, and (b) the integral of suchmeasurementadeviations and (c) the first time-derivative of themeasurement, namely, the

time-rate of change-lof-deviation.

FIGURE 2 represents a perspective view of a 2-function orproportional-reset controller embodying the pres ent invention, namely,one whose pneumatic output is substantially proportional to (a) thedeviation of meas urement from the predetermined setting and (b) the integral of the measurement-deviations.

FIGURE 3 represents a perspective view of a wideband proportionalcontroller embodying the present invention.

FIGURE 4 represents a perspective view of a narrowband proportionalcontroller embodying the present invention.

FIGURE 5 represents a perspective view of an on andoff controllerembodying the present invention.

FIGURE 6 represents a perspective view of a Z-function controller likethat of FIGURE 2, with an adjustable ratio input mechanism like that ofBrewer Patent 2,481,- 496, adapted to receive two input-motions and totransfer a single input-motion to the controller in proportion to thedeviation from a set ratio of the first two input-motrons.

FIGURE 7 represents a front elevational view of the 3- functioncontroller shown in FIGURE 1.

FIGURE 8 rep-resents a vertical cross-sectional view, taken through thecenter, of the controller shown in FIG- URE 1, in a plane parallel tothe plane of FIGURE 7, and taken generally on line 8-8 of FIGURE 9.

FIGURE 9 represents a horizontal sectional view taken on line 9-9 ofFIGURE 8.

FIGURE 10 represents a vertical sectional view taken on line 10'10 ofFIGURE 7.

FIGURE 10-a is a fragmentary cross-sectional view, on an enlarged scale,of the filter and constrictor-carrying plug shown at the bottom, left,in FIGURE 10.

FIGURE -h represents a fragmentary cross sectional view, on an enlargedscale, of the inlet and vent valves (of the relay) as shown at thebottom, center, in FIG- URE 10.

FIGURE 11 represents a fragmentary vertical cross-sectional view takenon line line 11-11-11-11 of FIG- URE 7 (but showing only the lefthalf'of such crosssection); showing, among other things, the feed-backpassage from the relay and the derivative needle-valve in said passage,and showing the reset passage and the reset needle-valve in saidpassage, but omitting the bellows and nozzle parts. FIGURE 12 representsa rear elevational view of the lower or relay section of the controller;showing the supply, ontput and feed-back air-passageways which registerwith corresponding passageways in the meeting or juncture face of theair-interconnector shown in FIGURES 13, 44 and 45 (and also showing thetwo screw-holes to receive the screws by which it is afiixed, withgasket interposed, to the relay-section).

FIGURE 13 represents a side elevation of the air-interconnector whichco-acts with the relay-section of the controller (FIGURE 10) shownimmediately adjacent to it and in operative alignment with it(cross-sections of this air-interconnector being shown in FIGURES 44 and45).

FIGURE 14 represents a vertical cross-sectional view of the Z-functioncontroller shown in FIGURE 2, taken in the same plane as that in whichFIGURE 11 is taken (but with a plug replacing the derivative valve ofthe controller shown in FIGURES 1, 710 and 11); this 'Z-functioncontroller being of the same construction as the 3- function controllershown in FIGURES 1, 7-13, except for the difference shown in this FIGURE14.

FIGURE 15 represents a rear elevation of the nozzlesection 104 of thecontroller, showing the rotatable flapper mounted thereon this form ofconstruction of nozzlesection 104 being common to all the controllersshown herein, excepting the on and off controller shown in FIG- URES 5and 46 to 53, inclusive.

FIGURE 16 represents a section on line 16-16 of FIGURE 15, showing theflapper-rotating shaft, gear and dial, journaled in the nozzle-section184.

FIGURE 17 represents a section on line 17-17 of FIGURE 15; on anenlarged scale (four times full size), showing the pilot or detectorvalve, including the sta tionary nozzle 187 and 193 and the tiltableflapper or baffle 225.

FIGURE 18 represents a section on line 18-18 of FIGURE 15 (on the samescale as FIGURE 17), showing said pilot or detector valve.

FIGURE 19 represents a front elevation of the setting input lever, on ascale approximately twice actual size.

FIGURE 20 represents a top plan view of the same.

FIGURE 21 represents a side elevation of the same.

FIGURE 22 represents a front elevation of the measurement input lever,on the same scale as FIGURES FIGURE 23 represents a top plan view of thesame.

FIGURE 24 represents a front elevation of the flapperdeflecting frame.

FIGURE 25 represents a top plan view of the same.

FIGURE 26 represents a side elevation of the same.

FIGURE 27 represents a front elevation of the flexure spring member.

FIGURE 28 represents a top plan view of the same.

FIGURE 29 shows the differential-adjustment screw 287 shown in FIGURES24 and 25, but here shown on an enlarged scale.

FIGURE 30 represents an axial cross-section of the derivative and resetneedle valves, on a scale approximately twice the actual size.

FIGURE 31 represents a front elevation of the wideband proportionalcontroller shown in FIGURE 3.

FIGURE 32 is a vertical center-line section of the controller as shownin FIGURE 31, taken in a plane parallel to the plane of FIGURE 31, online 32-32 of FIG- URE 33.

FIGURE 33 represents a horizontal section on line 33-33 of FIGURE 32.

FIGURE 34 represents a vertical section on line 34-34 of FIGUREv 31.

FIGURE 35 represents a fragmentary vertical crosssectional view, on line35-35 of FIGURE 31, of the left half of such section (omitting also thenozzle and flapper mechanism).

FIGURE 36 represents a top plan View of the controller shown in FIGURES31 to 35, inclusive.

FIGURE 37 represents a vertical section on diagonal line 37-37 of FIGURE36, of the cover section of the controller as shown in FIGURES 31 and 32to 35.

FIGURE 38 represents a fragmentary cross-sectional view similar to thatshown in FIGURE 35, of the left half of such section, showing a variantform of construction in which a set-screw 341 and a lock-nut 342 areused instead of the dial-knob (3'40 and 337 in FIGURES 3, 31, 35 :and36), for manual reset, and with derivative valve (shown in FIGURE 30)added, to form a proportional derivative action controller.

FIGURE 39 represents a front elevation of the controller shown in FIGURE4.

FIGURE 40* represents a center section of the controller shown inFIGURES 4 and 39, taken generally on line 40-40 of FIGURE 41.

FIGURE 41 represents a horizontal section taken generally on line 41.-41of FIGURE 40*.

FIGURE 42 represents a vertical section on line 42- 42 of FIGURE 39.

FIGURE 43 represents a fragmentary vertical cross sectional view on line43-43-43-43' of FIGURE 39, without bellows or nozzle mechanism; showingthe lefthand of such cross-section.

FIGURE 44 represents a section on line 44-44-44- 44 of FIGURE 45.

FIGURE 45 represents a section on line 45-45 of FIGURE 13.

FIGURE 46 represents a front elevation of the on-andoff controller shownin FIGURE 5.

FIGURE 47 represents a vertical section of the controller shown inFIGURE 46.

FIGURE 48 is a horizontal section on line 48-48 of FIGURE 46.

FIGURE 49* is a top plan view of the nozzle and flapper system of theabove controller.

FIGURE 50 is an elevation of the same.

FIGURE 51 is a top plan view of the flapper assembly. FIGURE 52 is anelevation of the same.

FIGURE 53 represents a horizontal section similar to that shown inFIGURE 48, but showing a reversal of the two housing-blocks which makeup this nozzle-and-flapper and lever section, whereby the controller maybe converted from a right-hand controller as in FIGURES 46, 47 and 48,to a left-hand controller as indicated in FIG- URE 53.

FIGURE 54 represents a top plan view of the housing or body portion 108of the relay section 101 shown in FIGURES 1 to 8, 10 to 12, 14, 31, 32,34, 35, 38, 39, 40, 42, 43, 46, 47, 81, 87 and 88.

FIGURE 55 represents a vertical section on line 55- 55 of FIGURE 54.

FIGURE 56 represents a fragmentary vertical crosssectional view taken ondiagonal line 56-56 of FIGURE 54; showing the half of said sectionthrough which line 56-56 is extended.

FIGURE 57 represents a top plan view of the partition and cover section102 shown inter alia, in FIGURES 1 to 7.

"FIGURE 58 represents a section on line 58-58 of FIGURE 57.

FIGURE 59 represents a section on line 59-59 of FIGURE 57.

FIGURE 60 represents a section on line 68-60 of FIGURE 57.

FIGURE 61 represents a top plan view of the bellows section of theproportional controllers (as, for example, bellows section 103 inFIGURES 1 to 5 and 7, and bellows section 185 in FIGURES l, 2, 6 and 7).

FIGURE 62 represents a front elevation of the same. FIGURE 63 representsa rear elevation of the same.

FIGURE 64- represents a bottom plan view of the same.

FIGURE 65 represents a section on line 6565 of FIGURE 64.

FIGURE 66 represents a section on line 6666 of FIGURE 64.

FIGURE 67 represents a section on line 6767 of FIGURE 61.

FIGURE 68 is a top plan view of the nozzle and flapper section 104 ofthe proportional controllers shown, inter alia, in FIGURES l-4, 6, 7, 9,10, 14, 15, 18, 31, 33, 34, 39, 41, 42, 87 and 89.

FIGURE 69 is a front elevation of the same.

FIGURE 70 is a bottom view of the same.

FIGURE 71 is a rear elevation of the same.

FIGURE 72 is a side elevation of the same.

FIGURE 73 is a horizontal section on line 73-73 of FIGURE 69. FIGURE 74represents a front elevation of the rear or mechanical input section ofthe proportional controllers shown, inter alia, in FIGURES 1-4, 6, 8, 9,32, 33, 40, 41, 81, 88 and 89.

FIGURE 75 represents a top plan View of the same.

FIGURE 76 represents a front elevation view of the nozzle and flappersection and of the mechanical input section of the on-and-oif controllershown in FIGURES 5, 46, 47, 48 and 53, the same section being used bothfor the nozzle and flapper at the front of the controller as well as forthe mechanical input at the back of the onand-offcontroller.

FIGURE 77 represents a bottom plan View of the same.

FIGURE 78 represents a rear elevation of the same.

FIGURE 79 represents a top plan View of the controller shown in FIGURES1, 7 and 8.

FIGURE 80 represents a bottom plan view (looking upward) of thecontroller last mentioned shown particularly in FIGURE 10.

FIGURE 81 represents a vertical section on line 81-81 of FIGURE 79.

FIGURE 82 represents a vertical section on line 82-82 of FIGURE 7 9. 1

FIGURE 83 represents -a front elevation of the ratio input mechanismshown at the top of the controller of FIGURE 6.

FIGURE 84 represents a horizontal section on line 84848484 of FIGURE 83.

FIGURE 85 represents a vertical section on line 8585 of FIGURE 84.

FIGURE 86 represents a vertical section on line 8686 of FIGURE 84.

FIGURES 87, 88 and 89, are similar to FIGURES 7, 8 and 9, respectively,and show the same controller, excep ting only that whereas thecontroller shown in FIG- URES 7, 8 and 9 (and also shown in FIGURE 1) isa right-hand controller, that shown in FIGURES 87, 88 and 89 is thelefthand counterpart thereof; showing the manner in which theinterchangeability of components permits the conversion of thecontroller from a left-hand controller to a right-hand controller,without the addition or subtraction of any part.

FIGURES 90 and 91 represent elevational views of lefthand and right-handdials, respectively, illustrating the construction and arrangement ofthe dials of the controllers of the present invention; the dials soillustrated being for the narrow-band proportional controllers.

FIGURES 92 and 93 represent elevational views of the ilefit-hand andright-hand dials, respectively, illustrating the construction andarrangement of the dials of the 6 controllers of the present invention;the dials so illustrated in these figures being for the wide-bandproportional controllers.

FIGURE 94 is a diagrammatic or schematic perspective view ot thecontroller shown in FIGURES 1, 7 to 13, 15 to 30, 54 to 78 and to 93.

The controllers :of the foregoing drawings and hereinafter more fullydescribed, are so constructed as to make the elements thereofinterchange able and separable, where by the same elements may be usedto form the minimumfunction controller, as, for instance, that shown inFIG- URES 5 and 46 to 48, inclusive, as well as to form themaximum-function controller, as for instance, that shown in FIGURES l, 7and 11, or to form any multiple-function controller such as those shownin the other draw-' ings and hereinafter more fully described.

Thus, the controllers of the present invention are characterized, interalia, by a sectional construction, with air-passageways formed in thesuccessive sections, in registration with each other and with thecooperative elements of each section in operative alignment with eachother, when the sections are operatively juxtaposed to each other.

The following is an outline of the principal sections of the contncllersof the present invention. The word sections as used in this descriptionhas reference to: the successive main components into which thecontrollers are divisible and out of which they are formed (asdistinguished from use of the word section to denote a crosssectional orsectional view or section as viewed in reference to drawings or graphicillustrations).

Referring to FIGURES l and 7 to 11, inclusive the controllers shown inthese figures include the relay section designated generally by thenumeral 101, the partition section 102, the proportional-belows section103, the input-and-nozzle section 104, the reset-bellows section 185 andthe cover section 186.

The input-and-nozzle section 194 may also be regarded as theinput-and-detector section, wherein the nozzlemember 187, theflapper-plate 2:25, and the flapper-deflecting means (together withtheir associated elements such as the detector-housing member 180',etc.) constitute the detector or the detector-section which forms a partof the input-and-detector section, and wherein the inputhousing member181 and the pivot, the levers and linkages carried thereby (FIGURE 9)constitute the input or the input section, which also forms a part ofthe inputand-detector or input-and-nozzle section 104.

The air-interconnector 187, as shown in FIGURE 13,

is, in turn, connected to the air-ported face 189 (FIG- URE 12) of therelay section 101, to form an interconnected passageway to connect theoutput passageway with the feed-back passageway, and to form and provideairsupply and air-output passageways communicating with thesepassageways in the relay. The relay section 101 (FIGURES l, 7, 8, 10, 11and l0b), which forms apart of each of the controllers hereinabovereferred to, includes a housing or main body member 188, in whose portedface 109 the air-supply port 118 and the output port 111 and thefeed-back port 112 are formed. The air-supply port is the terminus, inthe relay body 188, of the supply passageway 113. A valve-passageway 114is extended within the body 188 transversely of the supply passageway113, which communicates with the output pressure chamber 115 through theinlet-valve-seat member 116 which is stationarily mounted or cast insitu in the body 108. The inletvalve-seat member 116 is provided widr anaperture extending therethrough, the outer periphery of which serves asa stationary valve-seat. A flexible diaphragm 117 is mounted within therelay body 108 to form a closurewall of the output pressure chamber 115;being mounted therein and held in pneumatically-sealed relation to thebody 188 by a clamping ring 118 secured to the body by screws 119, .atspaced intervals around the ring.

The diaphragm 117 (FIGURE 10b) has a central

1. A SECTIONAL PNEUMATIC CONTROLLER INCLUDING A STACK OF SECTIONS,OPERATIVELY JUXTAPOSED AND DETACHABLY SECURED TO EACH OTHER IN SUCHSTACKS, THE SAID SECTIONS BEING REVERSIBLE WITHIN THE STACK SO AS TOCONVER THE CONTROLLER FROM LEFT-HAND CONTROLLER TO THE RIGHT-HANDCONTROLLER OR VICE VERSA, ONE OF SAID SECTIONS BEING ANINPUT-AND-DETECTOR SECTION AND AT LEAST ONE OF SAID SECTIONS BEING AFEEDBACK SECTION WHICH IS ALSO REVERSIBLE THROUGH A 180*